A polyarylene sulfide (hereinafter, also referred to as “PAS”), represented by polyphenylene sulfide (hereinafter, also referred to as “PPS”), is an engineering plastic having excellent heat resistance, chemical resistance, flame retardancy, mechanical strength, electrical characteristics, dimensional stability, and the like. The PAS has been widely used in a wide variety of fields of electric/electronic devices, devices for automobiles, and the like, since the PAS can be formed into various molded products, films, sheets, fibers, and the like by ordinary melt processing methods such as extrusion molding, injection molding, and compression molding.
As a representative method of producing a PAS, a method in which a sulfur source and a dihalo aromatic compound such as p-dichlorobenzene (hereinafter, also referred to as “pDCB”) are subjected to a polymerization reaction in an organic amide solvent such as N-methyl-2-pyrrolidone (hereinafter, also referred to as “NMP”) has been known.
With expanded application of a PAS, particularly with spread application of a molded product by melt processing, for example, a PAS having a high degree of polymerization, specifically a PAS having a high melt viscosity has been required as a PAS having excellent product characteristics, formability, and the like. A PAS composition that contains a PAS obtained by a polymerization reaction in the presence of a polyhaloaromatic compound having three or more halogen substituents in each molecule and has excellent organic solvent resistance (Patent Document 1) and a branched PAS which is obtained by a polymerization reaction in the presence of the polyhaloaromatic compound and has excellent spinning and stretching characteristics, and the like (Patent Document 2) have been known. In Patent Documents 1 and 2, the polyhaloaromatic compound is contained in an initial stage or a former polymerization step of the polymerization reaction.
With increased demand for a PAS, increase in yield during production of the PAS has been also required. Patent Document 3 discloses a production process of a PAS including a step of adding an aromatic compound such as a dihalo aromatic compound and a trihalo aromatic compound to a liquid phase in a polymerization reaction system after a phase-separated polymerization step, and a step of cooling the liquid phase, by which a granular PAS can be obtained at a high yield while retaining a melt viscosity at a high level. Specifically, Patent Document 3 describes a method in which pDCB which is a dihalo aromatic compound is added in an amount of 1.3 mol relative to 100 mol of NMP after completion of a second-stage polymerization (after a phase-separation step), and the liquid phase is slowly cooled by controlling a cooling rate to 0.8° C./min at a temperature of from 260° C. to 220° C., and then cooled to around room temperature without temperature control.
Patent Document 3 also discloses that the amount of waste such as oligomers and a finely particulate PAS can be reduced with increase in yield. As concern about environmental problems has grown, a method capable of further effectively reducing the generation of byproducts such as oligomers and a finely particulate PAS, which need to spend high cost and many processes for a detoxification treatment of industrial waste, has been required as the production method of a PAS, in addition to further increase in yield of a PAS having a high degree of polymerization.
Therefore, provision of a method of producing a PAS by a polymerization reaction of a sulfur source and a dihalo aromatic compound in an organic amide solvent, in which a PAS having a high degree of polymerization can be obtained at a high yield and the generation of a byproduct that imposes a large burden of a waste treatment and the like can be effectively reduced, has been required.